Microbial fuel cell with flexible substrate and micro-pillar structure

ABSTRACT

A microbial fuel cell includes a bio-compatible body having a micro-pillar structure defining an anode compartment adapted to contain a catalyst that metabolizes glucose to generate electrons and protons. A nano-porous membrane prevents loss of the catalyst from the anode compartment, while providing fluid access for ingress of glucose fuel and egress of waste.

This application claims priority from U.S. Provisional Application Ser.No. 60/717,504 filed Sep. 15, 2005.

FIELD OF THE INVENTION

The present invention relates to a new structure for a microbial fuelcell that uses a flexible substrate and micro-pillars.

BACKGROUND OF THE INVENTION

Implantable devices require power source for functional operation. Forexample, pacemakers have been widely used to stimulate heart muscles andlithium batteries are used to provide power. Emerging technologies suchas MEMS (MicroElectroMechanical Systems) promise to improve the qualityof life for patients suffer from chronicle diseases. Implantable sensorsmade by MEMS technology have the advantages of low-cost, small-in-size,easy integration with the controlling integrated circuits (ICs) and lowpower consumption. However, the size of conventional lithium batteriesis large compared to the MEMS-based sensors and post-processing willoften be needed to integrate the battery with the sensors.

Published United States Patent Application 20040241528 (Chiao et al2004) describes a MEMS-based microbial fuel cell in which a siliconsubstrate is used to fabricate miniature parallel fluid channels havinga current collecting coating. Yeast is used as a catalyst. The fuel isglucose and is found in abundance within the human body. When yeastmetabolizes glucose, electrons and protons are generated. Electrons arestored in biomolecules, such as NAD, the electrons are transferred tothe anode by oxidation/reduction reactions. On the other hand, protonsare diffused through a proton exchange membrane (PEM) and collected bythe cathode. Electric power can be harvested by connecting the anode andcathode through a dissipating device. The application of the MEMS fuelcell is to provide a powering device that is long-lasting, self-sustain,small-in-size and easy integration with ICs and MEMS sensors.

SUMMARY OF THE INVENTION

According to the present invention there is provided a microbial fuelcell, which includes a bio-compatible body having a micro-pillarstructure defining an anode compartment adapted to contain a catalystthat metabolizes glucose to generate electrons and protons. Anano-porous membrane prevents loss of the catalyst from the anodecompartment, while providing fluid access for ingress of glucose fueland egress of waste.

The above described MEMS-based microbial fuel cell is fabricated usingpolymeric materials. It has a larger surface-area-to-volume ratio thatcan increase the power output at least 4.5 times more than previouslyreported fuel cells. The advantages of the new fuel cell can besummarized below: (1) flexible substrate minimizes damage to the humanbody; (2) larger surface-area-to-volume ratio using a “micro-pillarstructure” improves power output and (3) integration with a nano-porousmembrane completely eliminates the need for micro-fluid ports.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will become more apparent fromthe following description in which reference is made to the appendeddrawings, the drawings are for the purpose of illustration only and arenot intended to in any way limit the scope of the invention to theparticular embodiment or embodiments shown, wherein:

FIG. 1 is an exploded perspective view of a microbial fuel cellconstructed in accordance with the teachings of the present invention.

FIG. 2A is a top plan view of an anode used in the microbial fuel cellof FIG. 1.

FIG. 2B is a top plan view of a cathode used in the microbial fuel cellof FIG. 1.

FIG. 2C is a detailed top plan view of the anode/cathode used in themicrobial fuel cell of FIG. 1.

FIG. 2D is a perspective view of the anode used in the microbial fuelcell of FIG. 1.

FIG. 3A through 3I is a side elevation view of the fabrication processfor fabricating the microbial fuel cell illustrated in FIG. 1.

FIG. 4 is a perspective view of a de-molding process for the microbialfuel cell illustrated in FIG. 1.

FIG. 5 is a perspective view of the microbial fuel cell illustrated inFIG. 1, with fluid ports and a quarter coin as a size comparison.

FIG. 6 is a schematic of the microbial fuel cell illustrated in FIG. 1,illustrating a micro-pillar structure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment, a microbial fuel cell generally identified byreference numeral 10, will now be described with reference to FIG. 1through 6.

FIG. 1 shows an exploded view of the proposed fuel cell 10.Bio-compatible polymeric materials, such as Polydimethylsiloxane (PDMS)is coated with a thin-film gold electrode and is used as the anode 12and cathode 14. Based on the morphology of target implantation sites,the PDMS anode 12 and cathode 14 can be fabricated to form a specificgeometry, thus can minimize damage to the body during implantation.Other bio-compatible polymeric materials can also be used. For example,polylactide-co-glycolide (PLGA) and polycarbonates are suitable for thisapplication. Furthermore, the PDMS 12/14 is shaped with a micro-pillarstructure 16, as shown for the cathode 12 in FIG. 2A through 2D.Referring to FIG. 6, the micro-pillar structure 16 is a column structurethat is typically 10 micrometer×10 micrometer×8 micrometer. Higheraspect-ratio micro-pillar structures can be made by shrinking thecross-sectional area and by increasing the height of each micro-pillar.Theoretically, the surface-area-to-volume ratio will increase with theaspect ratio of the micro-pillar structure 16. Referring again to FIG.1, in order to keep the yeast inside the anode compartment 18 duringoperation, a mechanical constraint is necessary, however, the glucoseand wastes must be allowed to move freely in and out of the fuel cell. Ananoporous membrane 20 (polycarbonate) is used to provide fluid accessof the glucose fuel and wastes (water and CO₂). Immunoisolation isprovided by the nanoporous membrane 20 since the pore size (10 nm) issmall enough to prevent yeast from leaking out from the anodecompartment 18. Antibodies will be blocked from entering the anodecompartment 18 due to the physical size of most antibodies (10-25 nm).

FIG. 3 shows the fabrication process of the PDMS anode 12 and cathode14. Micro-pillar structures 16 are formed by a process similar to thesoft lithography process using a silicon mold. First, referring to FIG.3A through 3C, 5000 Å silicon dioxide 22 is grown on silicon substrate24, and is patterned using a plasma-assisted etching process. Referringto FIG. 3D, silicon substrate 24 is then etched at room temperatureusing XeF2 and 800 mT N₂ gaseous mixture. Referring to FIG. 3E, thesilicon dioxide mask 22 is then removed in BOE. The silicon substrate 24serves as a mold and has complementary patterns of the micro-pillarstructure 16. Referring to FIG. 3F, the silicon wafer 24 is coated by aCr(500 Å) and Au(2000 Å) coating 26. This gold thin-film layer serves asa “sacrificial” layer that enhances the later PDMS demolding process.Referring to FIG. 3G, premixed 10:1 ratio PDMS (DuPont Sylgard 184) 28is spin-coated onto the silicon mold 24. After 1 hr degassing in avacuum chamber, it is cured at 60° C. for 2 hrs. Referring to FIG. 3H,during the de-molding process, it was found that the sacrificial goldthin-film increased the yield significantly, which has not beenpreviously reported in the literature. Experimentally, it was found thatthe deposition rate of the sacrificial gold film has a significanteffect on the de-molding process. For example, the deposition rate lessthan 2 A/sec can create a smooth surface that does not adhere to thePDMS well and the de-molding process will occur smoothly. If thedeposition rate is more than 2 A/sec, rough gold surface will be createdand PDMS adheres onto the gold surface well, as such rupture in the PDMSoften occurs during the de-molding process and the PDMS is no longeruseful. Lastly, fluid accesses 30 are mechanically punched and,referring to FIG. 31, 2500 Å Cr/Au 32 is evaporated onto the surface ofthe PDMS 12 or 14 as a conducting electrode.

FIG. 4 shows the de-molding process, with the PDMS 12 or 14 being peeledfrom the silicon substrate 24.

FIG. 5 shows the PDMS substrate 12 or 14 evaporated with Cr/Au 32, nextto a Canadian quarter 36 for an approximate size comparison. Referringto FIG. 1, the PDMS anode/cathode 12 and 14 is then assembled with a PEM(proton exchange membrane) 34 and a nanoporous membrane 20 is then gluedto the fluid access port 30, after dry yeast is packed into the anodecompartment 18.

FIG. 6 shows a SEM (Scanning Electron Microscope) microphoto of themicro-pillar structure 16 on the PDMS surface. Micro-pillar structures16 with an average height of 8 micrometer and a minimum feature size of10 μm are fabricated. The space between each micro-pillar is 1 μm. Thetotal number of micro-pillar structure is close to 54,000 in a 1.2 cm×1cm area. The surface-area-to-volume ratio increases 3 to 5 times in theanode and cathode compared to previous published work and can be easilyincreased more, if micro-pillar structure is made higher and thinner.

Experimental Results

Table 1 below shows the experimental results based on two fuel cells.The first fuel cell (Microchannel MEMS-MFC) is made of conventionalsilicon substrates with no micro-pillar structure but bulk etchedmicrochannels [Chiao et al, IEEE MEMS 2003]. The second fuel cell(Micro-pillar enhanced MEMS-MFC) is based on a flexible membrane withmicro-pillar structures. The two fuel cells are tested under closelyidentical experimental conditions. Discharge experiments through variousresistors were carried out to demonstrate the feasibility andimprovement of the micro-pillar structure over existing approach. It isshown that the power density generated by the Micropillar enhancedMEMS-MFC has a better power performance than the Microchannel MEMS-MFC.

Further experiments were conducted at 37±0.5° C. to simulate thecondition of normal human body temperature. Dehydrated yeast species,Saccharomyces cerevisiae, was cultured in pH 7.0 anolyte at 37° C.aerobically. D(+) Glucose with 1M concentration was instantly mixed withthe yeast culture after 12 hr incubation. 15 μL bio-catalyzed fuel wasthen injected to the anode compartment of the fuel cell for an houroperation. Typical constituents of anolyte and catholyte are tabulatedin Table 2 below. Chemically fed microbial fuel cell is situated in the37° C. aerated isothermal environment monitored by K-type thermal couple(TL180, Circuit Test Electronic) plugged in to the microprocessorthermometer (HH23, Omega) with 0.4% accuracy. Voltage and currentcharacteristics of the fuel cell were recorded by data acquisitionsystem (NI 4070, 1 μV and 10 nA resolution) via Lab View 7.1 programmingat 2 Hz sampling rate. TABLE 1 Performance characteristics of previousmicrochannel MEMS-MFC and the micropillar structure MEMS-MFC AverageMicrobial OCV Load Duration Current density Power density CellBiocatalyst Anolyte Substrate Mediator [mv] [Ω] [min] [μA/cm²] [μW/cm³]Microchannel S. cerevisiae Phosphate glucose Methylene 343 10 1415_((max)) 0.5 MEMS- buffer (1M) Blue (43 min) 50 5 1.3_((max)) — MFC(pH 7.0, (10 mM) 160 μL) Micropillar Phosphate glucose Methylene 420 4760 29.2_((max)) 3.9_((max)) enchaned buffer (1M) Blue (60 min)8.2_((60 average)) 0.4_((60 average)) MEMS-MFC (10 mM)14.2_((14 average)) 1.0_((14 average)) pH 7.0, 100 60 34.0_((max))11.1_((max)) 15 μl, 5.1_((60 average)) 0.5_((60 average)) @ 37° C.10.5_((14 average)) 1.5_((14 average)) 470 60 20.8_((max)) 19.5_((max))2.7_((60 average)) 0.6_((60 average)) 5.9_((14 average))2.2_((14 average)) 1000 60 17.4_((max)) 29.1_((max)) 1.6_((60 average))0.5_((60 average)) 3.4_((14 average)) 1.9_((14 average))

TABLE 2 Chemical constituents in a MEMS microbial fuel cell. ChemicalsConcentration Supplier Anolyte (pH 7.0, aqueous solution) Potassiumphosphate Dibasic 0.1M Fisher Scientifics Anhydrous (BP363-500)Potassium phosphate Monobasic 0.1M Fisher Scientifics (P285-500)Methylene blue 10 mM Fisher Scientific (M291-25) Catholyte (pH 7.0aqueous solution) Potassium phosphate Dibasic 0.1M Fisher ScientificsAnhydrous (BP363-500) Potassium phosphate Monobasic 0.1M FisherScientifics (P285-500) Potassium ferri(III)cyanide 20 mM Acros Organics(AC424125000) Fuel D(+) Glucose 1M Acros Organics (AC41095-0010)Catalyst Yeast Saccharomyces cerevisiae 0.85 g (dry weight) Sigma in 10ml Anolyte (YSC1-100G)

To quantify the effect of biocatalyst concentration for the fuel cellpotential development, budding yeast, S. cerevisiae cultured with 4 to14 hours was used for open circuit potential (OCV) measurement. A 15 μldrop of anolyte mixed with 1M glucose concentration was injected to theanode compartment for experiment.

Table 3 shows the average open circuit potential (OCV) versus yeastculturing time. The average OCV increased gradually and saturated after12 hours incubation with 420 mV. Catalytically, this result implicatesthe amount of biocatalyst directly related to the potential developmentfor the microbial fuel cell while the saturation is the consequence ofthe electrochemical nature of the fuel cell as well as the physiology ofS. cerevisiae inside the fuel cell compartment.

Table 3: The effect of incubation time of s. cerevisiae on the microbialfuel cell average open circuit voltage for an hour operation. 85 mg/mldehydrated S. cerevisiae was cultured in 37° C. in pH 7.0, potassiumphosphate buffer with 10 mM Methylene Blue. The open circuit voltage wasmeasured when 1 M D-glucose was mixed with the cultured microbialsolution.

Table 4 shows the correlation of glucose concentration with OCV. Variousglucose concentrations form 2.5 mM to 1M were adopted for the test withfixed 12 hours S. cerevisiae culturing. A strong logarithmic relation isreported between the average OCV and glucose concentration.

Table 4: Influence of glucose concentration on the open circuit voltage.85 mg/ml dehydrated S. cerevisiae was cultured in 37° C. in pH 7.0,potassium phosphate buffer with 10 mM Methylene Blue for 12 hr.

Based on the former experimental results, subsequent experiments wereconducted with fixed incubation time of 12 hours and 1M glucoseconcentration. Table 5 shows the evidence of the electron mediator,Methylene blue (MB), efficiently shuttling metabolic electrons from themicrobial body to the electrode surface. Samples with MB showed an OCV 4times greater than samples without MB.

Table 5: Open circuit potential development of the microbial fuel cellsunder different anolyte constituents. Typical catolyte consists of 20 mMpotassium ferricyanide in pH 7.0 potassium phosphate buffer.

Table 6 shows the current density of the fuel cell. Resistors (47Ω,100Ω, 470Ω and 1000Ω) were connected to the MEMS fuel cell individuallyfor one hour discharging time. A control experiment of 100Ω dischargingwith anolyte solely consisted of 1M glucose solution in pH 7.0 phosphatebuffer was used to demonstrate the effect of glucose being bio-catalyzedfor electricity generation.

Table 6: The performance of current density powered by single drop of 15μl anolyte with 1M glucose solution.

Table 7 shows the power density versus current density and Table 8illustrates the average power density generation under differentdischarge loadings in a certain period of time.

Table 7: The corresponding power density based on the current densitychange under 47Ω, 100Ω, 470Ω and 1000Ω resistors.

Table 8: The characteristics of microbial fuel cell average powerdensity powered by single drop of 15 μl anolyte with 1M glucose solutioncatalyzed by S. cerevisiae.

CONCLUSION

As described above, we have demonstrated that a fuel cell can be made ofbio-compatible polymeric materials, such as PDMS. The fuel cell, asdescribed above, is flexible. This flexibility is of benefit inminimizing damage to the human body both during and after installation.The fuel cell has higher surface-area-to-volume ratio compared toprevious published work. The fabrication process of the micro-pillarstructure using a gold sacrificial layer is novel. Experimental resultsshow fuel cells with micro-pillar structure can produce more power thanthe previous microchannel type silicon fuel cells.

Furthermore, previously, the micro channel based MEMS fuel cell couldonly generate 0.50μ W/cm³ for a 10Ω load [Chiao et al, IEEE MEMS 2003].MEMS fuel cell with a micro pillar structure can provide 4.5 times morepower density output at an average of 2.2μ W/cm^(3 for a) 470Ω loadingin the same time duration (Table 1). Furthermore, previous micro channelbased fuel cell generated 1iA/cm² current. The MEMS fuel cell with amicro pillar structure generates a peak 29.2μ A/cm² and average of 8.2μA/cm^(2 for) 60 minutes operation.

In this patent document, the word “comprising” is used in itsnon-limiting sense to mean that items following the word are included,but items not specifically mentioned are not excluded. A reference to anelement by the indefinite article “a” does not exclude the possibilitythat more than one of the element is present, unless the context clearlyrequires that there be one and only one of the elements.

It will be apparent to one skilled in the art that modifications may bemade to the illustrated embodiment without departing from the spirit andscope of the invention as hereinafter defined in the claims.

1. A microbial fuel cell, comprising: a bio-compatible first body have amicro-pillar structure defining a cathode; a bio-compatible second bodyhaving a micro-pillar structure defining an anode compartment adapted tocontain a catalyst that metabolizes glucose to generate electrons andprotons; and a nano-porous membrane preventing loss of the catalyst fromthe anode compartment, while providing fluid access for ingress ofglucose fuel and egress of waste.
 2. The microbial fuel cell as definedin claim 1, wherein the first body and the second body are made from aflexible polymeric material.
 3. A method of fabricating a microbial fuelcell having a micro-pillar structure, including: forming a mold for ananode micro-pillar structure in a silicon substrate; coating the moldwith a sacrificial gold film with a deposition rate of less than 2A/sec; distributing a thin layer of bio-compatible polymeric materialsinto the mold; curing the polymeric materials; forming conductingelectrodes and fluid access ports on the polymeric materials; assemblingthe anode micro-pillar structure with a cathode and a proton exchangemembrane; placing a catalyst that metabolizes glucose within the anodemicro-pillar structure; covering the fluid access ports with anano-porous membrane adapted to prevent loss of the catalyst whilefacilitating fluid access through the fluid access ports for the ingressof glucose fuel and egress of waste.